1. Field of the Invention
The present invention relates to a polymerizable compound suited to the manufacturing of a holographic recording medium, especially a volume holographic recording medium. The present invention further relates to an optical recording composition comprising the above polymerizable compound, a holographic recording medium formed with the above optical recording composition, and a method of recording information on the above medium.
2. Discussion of the Background
Holographic optical recording media based on the principle of the holograph have been developed. Recording of information on holographic optical recording media is carried out by superposing an informing light containing image information and a reference light in a recording layer comprised of a photosensitive composition to write an interference fringe thus formed in the recording layer. During the reproduction of information, a reference light is directed at a prescribed angle into the recording layer in which the information has been recorded, causing optical diffraction of the reference light by the interference fringe which has been formed, reproducing the informing light.
In recent years, volume holography, and, more particularly, digital volume holography, have been developed to practical levels for ultrahigh-density optical recording and have been garnering attention. Volume holography is a method of writing interference fringes three-dimensionally by also actively utilizing the direction of thickness of an optical recording medium. It is advantageous in that increasing the thickness permits greater diffraction efficiency and multiplexed recording increases the recording capacity. Digital volume holography is a computer-oriented holographic recording method in which the image data being recorded are limited to a binary digital pattern while employing a recording medium and recording system similar to those of volume holography. In digital volume holography, for example, image information such as an analog drawing is first digitized and then expanded into two-dimensional digital pattern information, which is recorded as image information. During reproduction, the digital pattern information is read and decoded to restore the original image information, which is displayed. Thus, even when the signal-to-noise (S/N) ratio deteriorates somewhat during reproduction, by conducting differential detection or conducting error correction by encoding the two-dimensional data, it is possible to reproduce the original data in an extremely faithful manner (see Japanese Unexamined Patent Publication (KOKAI) Heisei No. 11-311936 or English language family member US 2002/0114027 A1, which are expressly incorporated herein by reference in their entirety).
For example, Published Japanese Translation of a PCT International Application (TOKUHYO) No. 2005-502918 or English language family member WO 03/023519, which are expressly incorporated herein by reference in their entirety, disclose the use of a urethane matrix and a phenyl acrylate derivative in a holographic optical recording medium of the photopolymer type. However, with common photopolymers, there is a major problem in that the polymerization of monomers is accompanied by volumetric shrinkage, distorting the recorded interference fringe and causing errors during the input/output of data, thereby compromising the precision of recording and reproduction (see Journal of the Japanese Society of Printing Science and Technology, 2004, Vol. 41, p. 25, which is expressly incorporated herein by reference in its entirety).
Published Japanese Translation of a PCT International Application (TOKUHYO) No. 2004-507513 or English language family member WO 02/19040, which are expressly incorporated herein by reference in their entirety, disclose the use of a cationic polymerizable monomer with little volumetric shrinkage as a recording material. However, the improvement in volumetric shrinkage is inadequate even when the technique described in Published Japanese Translation of a PCT International Application (TOKUHYO) No. 2004-507513 is employed.
Japanese Patent No. 3504884 or English language family member U.S. Pat. No. 6,124,076, which are expressly incorporated herein by reference in their entirety, disclose a method for improving volumetric shrinkage by adding a volume-expanding agent. However, in the method described in Japanese Patent No. 3504884, since the wavelength of the light employed in the reaction of the volume-expanding agent is identical to that of the recording light, in principle there is a problem in the form of reduced recording sensitivity. Accordingly, there is a need to develop a holographic recording medium of high recording and reproducing precision in which it is possible to compensate for volumetric shrinkage.